The invention relates to a low-pressure alkali metal vapor discharge lamp provided with a discharge envelope or arc tube which is arranged in an evacuated outer bulb, electrodes arranged in the arc tube and incorporating an emitter containing an alkaline earth metal oxide, and a getter in the evacuated space. As used hereinafter, an alkali metal vapor discharge lamp refers to one in which, in an operating condition, the arc discharge current is carried primarily by an alkali metal vapor rather than by an inert gas. A low-pressure sodium vapor lamp of this kind is known from German Pat. No. 913,468.
The known lamp has a gettering metal layer deposited on a wall in the evacuated space between the outer bulb and the discharge envelope. This gettering metal layer serves to bind residual gases and gases released during the life of the lamp. Thus a high vacuum is maintained which minimises thermal losses which would otherwise result from heat conduction by these gases. Examples of such gases are: hydrogen, oxygen, nitrogen, water vapour, carbon monoxide and carbon dioxide.
The gettering metal layer may consist of a metal, such as barium, calcium, strontium, or magnesium. The metal layer is obtained by heating an open holder provided with such a metal after the outer bulb has been evacuated, as a result of which the metal evaporates for the major part and is deposited on a wall opposite to the opening of the holder.
In general, the getter metal is present in the holder in the form of an alloy with, for example, aluminum. In this form, the gettering metal can be manipulated more readily in air. The alloy can be mixed with a metal powder, such as nickel, iron, titanium or thorium powder, for example with an equal quantity by weight of such a powder. Such a powder enters into an exothermal reaction with the alloy upon heating, as a result of which the getter metal is released and evaporated more rapidly. Mostly, a part of the gettering metal remains in the holder, generally as a compound with, for example, oxygen or with the metal with which the getter metal was provided as an alloy.
Otherwise, a holder with a non-evaporating getter, for example, a metal strip coated with zirconium aluminium or zirconium nickel, may be present in the evacuated space.
Before the discharge envelope is sealed, this envelope is evacuated, while the electrodes are heated in order to release absorbed gases therefrom. Carbon dioxide may be produced at this time if the emitter is provided on or in the electrode in the form of one or more carbonates, or water may be produced if the emitter is provided in the form of a hydroxide. Subsequently, the discharge envelope is provided with its gas filling and sealed.
It has been found that during their heating the electrodes lose a part of the quantity of emitter, which deposits on the wall of the discharge envelope. This loss of a part of the supply of emitter of the electrodes is contrary to the endeavours made to furnish the electrodes with as large a quantity of emitter as possible, in order to prevent the lack of emitter from curtailing the life of a lamp.
It has further been found that lamps have a higher ignition voltage if the electrodes are heated during the manufacturing process only to a temperature at which just no loss of emitter occurs.
An example of a getter-like action in an alkali metal arc tube is shown in British Pat. No. 2,007,423, which describes a low-pressure sodium vapor discharge lamp which contains in the arc tube a substance which binds potassium. This substance, sodium iodide, is not so much a getter as an ion exchanger. Potassium, which is released from the wall of the discharge vessel, enters into a displacement reaction with this substance, in which the sodium ion from the iodide is replaced by a potassium ion and sodium is released (NaI+K.fwdarw.KI+Na).